Battle Calculator: Mimic legacy behaviour for intial setup

Mimic the legacy behaviour and force the two parties in the initial
setup to be non-allied.  This legacy behaviour was requested in #9532.

game-app/game-core/src/main/java/games/strategy/triplea/odds/calculator/BattleCalculatorPanel.java

@@ -1095,6 +1095,16 @@ class BattleCalculatorPanel extends JPanel {
           }
         }
 
+        // Mimic the legacy behavior and force the parties to be non-allied modifying either
+        // attacker or defender like the old code did). This legacy behavior was requested in #9532.
+        if (data.getRelationshipTracker().isAllied(getDefender(), getAttacker())) {
+          if (location.isWater() && players.isEmpty()) {
+            getEnemy(getAttacker()).ifPresent(this::setDefender);
+          } else {
+            getEnemy(getDefender()).ifPresent(this::setAttacker);
+          }
+        }
+
         setAttackingUnits(
             location.getUnitCollection().getMatches(Matches.unitIsOwnedBy(getAttacker())));
         setDefendingUnits(
@@ -1361,6 +1371,27 @@ private boolean isLand() {
     return landBattleCheckBox.isSelected();
   }
 
+  /**
+   * Get a suitable enemy for {@code player}.  First checks for players at war, if none found, checks
+   * for non-allied players, if still no foe was found, returns an empty Optional.
+   *
+   * @param player the player to find a foe for
+   * @return Optional with not an ally or an empty Optional
+   */
+  private Optional<GamePlayer> getEnemy(final GamePlayer player) {
+    for (final GamePlayer gamePlayer : data.getPlayerList()) {
+      if (data.getRelationshipTracker().isAtWar(player, gamePlayer)) {
+        return Optional.of(gamePlayer);
+      }
+    }
+    for (final GamePlayer gamePlayer : data.getPlayerList()) {
+      if (!data.getRelationshipTracker().isAllied(player, gamePlayer)) {
+        return Optional.of(gamePlayer);
+      }
+    }
+    return Optional.empty();
+  }
+
   private void setResultsToBlank() {
     final String blank = "------";
     attackerWin.setText(blank);